Alum is a crystalline, astringent substance with a sweetish taste. It is a double sulphate of potassium and aluminium with water of crystallization. It crystallizes in colourless regular octahedra. Its solution reddens vegetable blues. When heated, its water of crystallization is driven off, and it becomes light and spongy with slightly corrosive properties, and is used as a caustic under the name of burnt alum.
Alum is prepared in Great Britain at Whitby from alum-slate, where it forms the cliffs for miles, and at Hurlet and Campsie, near Glasgow, from bituminous alum shale and slate-clay, obtained from old coal-pits. It is also prepared near Rome from alum stone. Common alum is strictly potash alum; other two varieties are soda alum and ammonia alum, both similar in properties. Iron alum (pale mauve) and chrome alum (deep purple) are compounds containing iron and chromium in place of aluminium.
Alum is employed to hardentallow, to remove grease from printers' cushions and blocks in calico manufactories; in dyeing as a mordant. It is also largely used in the composition of crayons, in tannery, and in medicine (as an astringent and styptic). Wood and paper are dipped in a solution of alum to render them less combustible. Research Alum
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Abbreviations
SciTech
Research Alum
